Certain classes of military munitions utilize the spinning motion of one or more air-deployed munitions to search within a target area for potential targets. After deployment at a height and relative position above and near the intended target(s), these munitions (also referred to as “submunitions”) operate as “top attack” weapons to detect, attack, and destroy stationary or moving targets from above. Common targets for these types of submunitions include tanks and other armored fighting vehicles. Such submunitions include a housing for a warhead, optical sensors, and electronics for image processing. The warheads of these submunitions typically are explosively formed projectiles. When a target has been detected by the optical sensors and identified by optical recognition software included within the image processing electronics, the submunition warhead is fired at the target. Such submunitions are often called sensor-fuzed submunitions, because the firing sequence is initiated or “fuzed” by the included optical sensors.
These submunitions are commonly dispensed from a suitable airborne carrier vehicle or may be fired from artillery. Various aerodynamic systems may be included onboard these submunitions to attain desired flight dynamics after deployment from the carrier vehicle or artillery. These aerodynamic systems typically operate to control the deceleration, orientation, and stabilization of the submunition, and may also impart spinning and coning motions to the submunitions as they fall toward the target area. As a result of these imparted spinning and coning motions, each field of view (FOV) of the optical sensors scans the underlying target area in an inwardly tightening spiral as the submunition descends. This inwardly tightening spiral scan pattern allows the sensors to “search” for a desired target within a given target area.
During the decent of the submunitions, deceleration, orientation, and stabilization functions are key to enabling successful operation of the submunition. To achieve deceleration, a decelerator is typically deployed after the submunition is dispensed from the carrier vehicle or piece of artillery. The decelerator provides at least two functions. One function of the decelerator is to slow down the submunition from its initial velocity. Another function of the decelerator is to re-position the submunition to a near vertical orientation during descent at a terminal velocity. The decelerator may also function to displace the spin axis of the submunition with respect to its principal axis to generate the desired inwardly tightening spiral scan pattern that is used for target search and acquisition.
One type of submunition decelerator is a single-bladed flexible wing that is attached to a spinning submunition. Examples of such decelerators are described in U.S. Pat. No. 4,635,553 to Kane and U.S. Pat. No. 4,756,253 to Herring et al, both of which are owned by the assignee of the present application. These single-blade decelerators are sometimes referred to as “samara blades”, or “samara wings”, in reference to the similarity to certain winged seeds (samara is Latin for “seed of the elm”).
FIG. 1 is a perspective view representing a simplified prior art spin-stabilized submunition 100 with a samara wing 102. The samara wing 102 is shown in stowed and deployed positions in FIG. 1A and FIG. 1B, respectively. The submunition 100 has a cylindrical housing 106 with a principal axis 108. The principal axis 108 is shown as substantially collinear with a spin axis 110 in FIG 1A and offset from an adjusted spin axis 110′ in FIG. 1B. One end of the samara wing 102 is connected to a root location of a base 104 located at one end of the submunition 100. After its deployment from a carrier vehicle or piece of artillery and prior to the deployment of the samara wing 102, the submunition 100 spins about spin axis 110 with an initial angular velocity (Ω) 112. In the stored position, the samara wing 102 is held in place inside of the periphery 105 of the spinning submunition 100 at a radial distance 120 from the principal axis 108.
In FIG. 1B, the samara wing is shown in a deployed position useful for the deceleration, orientation, and stabilization of the submunition 100 while it is spinning in flight. When the samara wing 102 is deployed from a spinning submunition 100, the samara wing 102 is held taut by the centripetal force acting on a mass, or “tip weight,” 102b that is located at one end of the samara wing 102 that is distal to the root location. As shown, the samara wing 102 has a desired width, or “chord” 102c, and a wingspan 102d. The main flight surfaces 102a of the samara wing 102 are positioned at a desired inclination angle, or angle of attack, to the relative wind stream as the submunition 100 spins in flight. When the samara wing 102 is deployed, the tip weight 102b has a tangential velocity, indicated by 114, that is related to the angular velocity (Ω′) 112′.
With continued reference to FIG. 1B, the force that the tip weight 102b exerts back on the submunition 100 through its attachment point on the top of the submunition 100 close to the periphery causes the submunitions 100 to spin about the adjusted spin axis 110′, which is shifted from the principal axis 108 of the submunition through an angle θ. This shifting of the of the spin axis 110′ from the principal axis 108 produces the desired scanning motion in which the precession rate and the spin rate (Ω′) 112′ of the submunition 100 are equal to one another. Because the optical sensors (not shown) onboard the submunition are aligned along the principal axis 108, the matching of the precession rate and spin rate allows the submunition 100 and sensor FOV to maintain the same orientation with respect to the ground along the direction of the principal axis 108.
During flight of the submunition, the deployed samara wing 102 produces aerodynamic lift in a direction along the spin axis 110′ of the submunition and opposite the direction of travel and thereby initially acts to decelerate the submunition 100. This deceleration acts through a center of drag that is displaced behind the center of gravity of the submunition 100. Consequently, as the submunition 100 loses its initial velocity, the acceleration of gravity causes the principal axis 108 to tip over toward a vertical orientation that is aligned with the flight path. Eventually, the acceleration due to gravity and the lift force become equal in magnitude and opposite in direction, causing the submunition 100 to achieve a terminal velocity. The samara wing 102 causes the submunition 100 to auto-rotate as it is pulled through the air and achieves a spin rate that results from the balance of the lift of the wing and its aerodynamic drag.
Samara wings have certain advantages over other types of decelerators. For example, the design parameters of a samara wing, e.g., wing span, chord, and tip weight mass, can be selected for different applications and conditions to yield a desired scanning pattern on the target area that leaves very little opportunity for the sensor trace, or scanned FOV, to miss any targets that may be present. Hence, the use of a samara wing in conjunction with a submunition can enable very effective lethality using simple optical sensors, e.g., those utilizing a small number of linear detector arrays. Further, samara wings may be used on any submunition that is dispensed or deployed at altitude and allowed to free-fall to earth. The submunitions can include mines or any variety of top attack smart submunitions.
While the operation of a samara wing can be simple and dependable once deployed, the requirements for the successful deployment of the samara wing 102 are not trivial and can be difficult to achieve. For example, if during the deployment of the samara wing, the tip weight 102b were to be simply released it would fly away with its initial tangential velocity. Absent an acceleration force to alter its angular velocity, the tip weight would fall behind and indeed wrap itself over the top of the submunition as the submunition spins, a condition that is known as wing-wrap.
Previous attempts have been made to address the problems of wing-wrap and variability of loading during deployment of a samara wing. Certain techniques utilize sacrificial rip stitching to releasably hold the samara wing in a folded, or “accordion-like” configuration. When the tip weight is released for deployment of the samara wing, the centripetal force developed at the tip weight pulls the rip stitching apart. Such techniques are passive in that they rely on the forces developed on the tip weight for the deployment of the samara wing. Because the flight dynamics and deployment conditions, e.g., atmospheric conditions, can vary drastically, passive deployment techniques have proven to be susceptible to a high degree of variability. Such passive techniques have been unreliable, with failed deployment of samara wing occurring in certain situations.